The present invention generally relates to the field of power supplies and more particularly to the field of power supplies that convert alternating current to low power, low voltage direct current.
In a typical AC power system in an aircraft, there are many low power low voltage control circuits that require a DC voltage directly from the AC lines. The AC line voltage generally provided by aircraft power systems usually has a frequency of 400 Hz at 115V. For variable frequency systems, the frequency is usually in the range of from 360 Hz to 800 Hz. In many cases, the power required for such low power low voltage control circuits is very small. However, in conventional AC/DC power supply design, the input capacitor required to interface the control circuit with the aircraft power supply system is a high voltage, high capacitance part, which tends to be bulky when compared to the size of other control circuit components. This dramatically increases the physical size of the AC/DC power supply.
This problem has been dealt with in several different ways. The most common approach before introduction of the switch mode power supply has been the circuit 100 shown in FIG. 1. Here, a simplified schematic of a line transformer 110 coupled AC/DC linear regulator 120 is shown. Generally, line transformers 110 are bulky and heavy since they are constructed with iron cores. Because of these disadvantages, this configuration has been replaced by the switch mode power supply design in many applications.
For low power applications, this circuit has been replaced by a flyback converter design. FIG. 2 shows the simplified schematic of circuit 200 having off-line flyback converter, which may be found commercially either as discrete components or as an integrated circuit. Since this circuit operates at a high frequency (usually in the order of hundreds of kilohertz), a transformer 210 may be much smaller than the line transformer coupled AC/DC linear regulator 120 shown in FIG. 1. However, the circuit 200 still suffers from at least two problems. First, an input capacitor 220 of the circuit 200 is bulky and keeps the physical size of the circuit large. Second, the input current is pulsed, which causes undesirable electromagnetic interference (EMI) emissions which interfere with other circuits that may be in close proximity. Suppression of EMI emissions is frequently required, but this complicates the circuitry and thus increases circuit size and part count.
Other designs have been promoted, which address different aspects of the problem. The disclosure of U.S. Pat. No. 4,665,355, issued to McCollum et al. describes an off-line capacitor divider power supply, which supplies regulated DC voltage levels referenced to the AC voltage source output terminal to a load such as the control and drive circuits of a solid state power controller. The power supply design is that of a linear power supply which is good only for a fixed frequency, fixed input voltage and a fixed load. When either the input voltage or the frequency changes, the design will suffer from poor efficiency, overheat, and loss of regulation.
The disclosure of U.S. Pat. No. 4,910,654, issued to Forge, describes an AC to DC conversion circuit, in which the first stage comprises a diode bridge rectifier coupled to a capacitor divider that in turn serves as input to a switch mode power supply. The disclosed circuit addresses the problem of burning out the current limiting resistor connected to the input terminal of the AC to DC converter stage of the switch mode power supply by removing the resistor and substituting a non-dissipative capacitive divider circuit which charges the filter or storage capacitors while the input relay is open. It does not address the problem of large value capacitors in the circuit.
The disclosure of U.S. Pat. No. 5,282,126, issued to Husgen, describes a starter circuit for a switched mode power supply, in which a capacitor divider is used to receive the pulsing DC voltage provided by a bridge rectifier. The use of the capacitor divider serves as both a smoothing device for the pulsing DC voltage and eliminates the need for transistor and collector resistance rated for high voltages and large currents. However, it does not address the problems of complexity, high part counts, and large value capacitors.
Hence, it can be seen that there is a need for a power supply for aircraft electrical systems in which the conversion circuit that is small in size. Associated with the space requirement, it is desirable to have as low a component count as possible to reduce inventory requirements for spare parts and to have high reliability to reduce the probability of failure at critical times. Furthermore, close proximity of the aircraft components with each other may promote interference between systems, so that low EMI emissions may also be desirable.